The detention of two prominent journalists in Turkey has provoked national and international criticism. The reporters were detained Thursday on spying charges by an Istanbul court following reports that Turkey's intelligence agency sent weapons to Islamist rebel groups in Syria.

The arrests are the latest in what is seen as a crackdown on media freedom following this month's landslide victory by the ruling AK Party.

Condemnation is growing over the jailing of two leading Turkish journalists. Kemal Kilicdarolgu, the leader of the main opposition Republican People’s Party, described the arrests as a black day for democracy and freedom of press.

The U.S. embassy, in a tweet, expressed great concern about what it said appears to another media outlet under pressure. The editor of Turkey’s Cumhuriyet newspaper, Can Dundar, and his Ankara bureau chief, Erdem Gul, are facing more than 20 years in jail on spying charges.

Dundar, speaking before his trial on Thursday, expressed defiance. He said he and Gul are journalists, not traitors or spies.

The journalists are being prosecuted for a story alleging Turkish intelligence ran guns to Islamist groups in Syria. Prosecutors claim the story is part of a conspiracy against the government by followers of the Islamic cleric Fethullah Gulen, once an ally of President Recep Tayyip Erdogan and now bitter rival.

The prosecution follows the personal intervention of Erdogan who warned the authors of the story would pay a heavy price.

Turkey researcher Andrew Gardner of the human rights group Amnesty International says the latest arrests are part of an increasing crackdown on dissenting media.

"In any normal circumstances, it would be shocking for these 2 journalists to be put in prison for really doing a very good job as journalists. But unfortunately in today’s Turkey its not surprising Its just another telling indictment on the terrible situation of freedom of expression and the pressure of the media," said Gardner.

Observers say the latest detentions come at an embarrassing time for the European Union, which is expected to unfreeze membership talks despite human rights concerns as part of an agreement with Turkey over refugees at a summit this weekend.